The present invention relates to an optical head for writing and focusing a light beam for reading information to and from an information recording medium such as an optical disk, and moe particularly, to an improvement in a photo detector for an optical head for detecting a defocusing state of the light beam.
Various optical information recording/reproduction systems have been recently devised which optically write and read information on and from an information recording medium (to be referred to as an optical disk hereinafter). For example, information recording/reproduction systems are known which are used for a recording medium for reproduction only such as a CD (compact disk) type of DAD or a video disk, an image file, a still image file, a COM (computer output memory) or the like.
In such systems, information is recorded by establishing a state change such as forming a hole (pit) in a recording layer by a focused light beam. In other information recording/reproduction systems, information is optically written on or read from an information recording medium for recording/reproduction or from an erasable information recording medium (to be referred to as an optical disk hereinafter). In these systems, in both the write and read modes, a light beam must be always focused on the optical disk. In other words, the beam waist of the light beam must be coincident with the optical disk surface, and a minimum-sized beam spot must be formed on the optical disk. In view of this requirement, the optical head frequently has a focus detecting system for detecting the state of focus of the light beam. Various such focus detecting systems have been proposed. For example, U.S. Ser. No. 399,873, filed on July 19, 1982 (now U.S. Pat. No. 4,521,680) and corresponding EPC application No. 82106508.3 filed on July 19, 1982 disclose a focus detecting system adopting the so-called knife edge method.
In this focus detecting system adopting the knife edge method, as shown in FIG. 1, a laser beam is reflected from a recording layer or light reflecting layer 10 on which information is to be or is already recorded. An objective lens 2, a knife edge 4, a convergent lens 6 and a photo detecting unit 8 are arranged on an optical path 0-0 of the reflected light from the recording layer 10. The knife edge 4 serves to allow passage of only that laser beam component which is spaced apart from the optical axis 0-0. The photo detecting unit 8 detects a laser beam which is converged by the convergent lens 6. The photo detecting unit 8 has two, first and second, photo sensing regions or photo sensitive regions 8-1 and 8-2, and is located at the back focal point of the convergent lens 6. In such a focus detecting system, the defocusing state is not detected by a change in the beam spot size on the photo detecting unit 8 but is detected by a deviation in the beam spot position in a direction 9. In the just in-focusing state, the beam spot is formed at the boundary between the two photo sensivity regions 8-1 and 8-2 of the photo detecting unit 8. A differential signal of the two photo signals from these regions 8-1 and 8-2 is kept substantially zero. In contrast to this, when the objective lens 2 draws too near to the recording layer 10 or is spaced too far apart therefrom, resulting in the defocusing state, the differential signal of the signals from the first and second photo sensitive regions 8-1 and 8-2 changes in the positive or negative direction. The level of this differential signal depends on the distance between the objective lens 2 and the recording layer 10, as shown in FIG. 2.
Referring to FIG. 2, the differential signal level is plotted along the ordinate and the distance from the just in-focusing state position is plotted along the abscissa. A direction away from the recording layer is designated as a positive direction, and a direction toward the recording layer is designated as a negative direction.
In the system described above, the following problem is presented. When the objective lens 2 is moved from the just in-focusing state position exceeding a distance .delta.a in the direction away from the recording layer, the beam spot on the photo detecting unit 8 is formed in the photo sensitive region 8-1, and a negative differential signal is generated by the photo detecting unit 8 in the same manner as in the case wherein the objective lens 2 is too close to the recording layer 10.
The above problem is also presented in a focus detecting system adopting an astigmatic optical system, in addition to the focus detecting optical system adopting the knife edge method. In the focus detecting system employing the astigmatic optical system, as shown in FIG. 3, a cylindrical lens 12 is arranged between a projection lens 6 and a photo detector or photo detecting unit 8, and a change in shape of the beam spot on the photo detector 8 is detected so as to determine whether an objective lens 2 is maintained in the just in-focusing state or the defocusing state. When the objective lens 2 is kept in the just in-focusing state, a circular beam spot is formed on the photo detector 8. However, when the objective lens 2 is kept in a defocusing state, an elliptical beam spot is formed on the photo detector 8. A focusing signal is generated in accordance with the shape of the beam spot. In such a system, since the projection lens 6 is combined with the cylindrical lens 12, these lenses have two focal points. A laser beam projected from the cylindrical lens 12 has two convergent points. Therefore, although the objective lens 2 is greatly removed from the light reflecting layer 10 and is kept in the defocusing state, a circular beam spot is formed on the photo detector 8, thus erroneously detecting that the objective lens 2 is kept in the just in-focusing state.